A Megascale
View of Reservoir Quality in Producing Sandstones from the Offshore
Ehrenberg, S. N.1, Paul H.
Nadeau2, Øyvind Steen3 (1) Statoil, N-4035 Stavanger,
Norway (2) Statoil ASA, N-4035 Stavanger, Norway (3) Statoil, Stavanger, Norway
A dataset (publicly available from the
U.S. Minerals Management Service, 2003) has been examined for relationships
between average values of porosity, permeability, depth, temperature, pressure,
thickness, age, and play type for 10,813 sandstone reservoirs (mostly Miocene
and younger) from the U.S. Gulf of Mexico (GOM). Porosity shows wide scatter as
a function of burial depth, but the mean (P50) porosity trend decreases
smoothly with depth and is much higher (10-13 porosity units) than the P50
porosity-depth trend for sandstone reservoirs worldwide (Ehrenberg and Nadeau,
2006). This difference is attributed mainly to the rapid rates of
sedimentation/subsidence for these young GOM reservoirs, most of which have
spent relatively little time above 80 Celsius where quartz cementation is
active. The effect of overpressure is apparent in plots showing displacement to
higher porosity for given depth and age of reservoirs with hard overpressure.
Multivariate regression analysis shows that porosity is best predicted by
temperature (rSQUARE=0.41), with the fit improved slightly by adding age and
then depth (rSQUARE=0.45). Grouping the reservoirs by chronozone reveals
regular trends of decreasing mean porosity and permeability with increasing
age, reflecting increasing mean depth and temperature with age. Permeability
shows clear trends of correlation of maximum and mean (P50) values with
porosity. The GOM P50 trend lies 0.2-0.6 permeability
log units below the P50 permeability trend for sandstone reservoirs worldwide,
probably reflecting fine grain size of most GOM sands. Permeability-porosity
trends show little variation for different ages and play types, reflecting
relative constancy of depositional sand quality throughout time and
depositional settings.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California